1. Field
The present invention relates generally to a catalytic device for cleaning and thermally managing a contaminated fluid, and more particularly to a catalytic device for use on a vehicle exhaust system.
2. Description of Related Art
Exhaust systems perform several functions for a modern engine. For example, the exhaust system is expected to manage heat, reduce pollutants, control noise, and sometimes filter particulate matter. Generally, these individual functions are performed by separate and distinct components. Take, for example, the exhaust system of a typical small gasoline engine. The small engine exhaust system may use a set of heat exchangers or external baffles to capture and dissipate heat and/or heat shields to protect the vehicle and/or the operator from excessive heat. A separate muffler may be coupled to the exhaust outlet to control noise, while a catalytic converter assembly may be placed in the exhaust path to reduce non-particulate pollutants. Although particulates may not generally be a concern in the small gasoline engine, some applications may benefit from the use of a separate particulate filter. Due to space limitations, costs, and engine performance issues, it is not always possible to include separate devices to perform all the desired functions, thereby resulting in an exhaust system that is undesirably hot, polluting, or noisy.
Known exhaust systems are often arranged with catalytic devices to support non-particulate emission control. Due to the physical size and reactivity requirements for these devices, their placement options are quite limited. Each device that must be placed adds additional design time, cost, and consumes valuable and limited space in the product. As emission requirements tighten, it is likely that more catalytic effect will be required, as well as further particulate control. In general, there has been a trend to place catalytic converters closer to the engine manifold in order to improve the transfer of heat to the catalysts and to decrease the time it takes for the catalysts to reach the operating or ‘light off’ temperature. However, it is not always possible to find a safe and effective placement for catalytic devices. Further, it is desirable and efficient for a for the amount of heat conveyed into the catalytic converter or a thermoelectric generator from the exhaust gas to be maximized and the waste heat lost to the surroundings to be minimized. Moreover, in the case of a typical catalytic converter, once they have begun, the catalytic reactions taking place are exothermic and can thus excessively heat the outside of the catalytic device housing assembly if not insulated properly. Such heating may pose human risk, such as burning the operator's hands or legs, as well as harm to the surrounding environment, if, for example, the heat causes dry grass to catch fire. These engines, such as small diesel or gasoline internal combustion engines (ICEs), are often found on motorcycles, lawn equipment, and recreational vehicles. Unfortunately, these small engines have generally not been able to benefit from catalytic technologies. In many applications, there is a need for a flexible, yet highly effective method to catalyze and remove the harmful emissions without producing excessive heat generation and transfer to the surrounding structure an/or environment. The ability to reduce noise pollution, as well as prevent injuries or harm due to excess heat is also desirable.
Known catalytic systems do not effectively operate until a threshold operational temperature is reached. During this “light-off” period, substantial particulate and non-particulate pollution is emitted into the atmosphere. Accordingly, it is often desirable to place a catalytic device close to the engine manifold, where exhaust gasses are hottest. In this way, the catalyst may more quickly reach its operational temperature. However, design or safety constraints may limit placement of the catalytic converter to a position spaced away from the manifold. In such a case, known exhaust systems have provided insulation on the inside of the pipe leading from the manifold to the catalytic converter. Again, similar constrains apply to the use of other devices that rely on engine heat for their operation, such as thermoelectric generation and electric power production. This insulation is used to direct heat from the manifold to the catalytic converter, where the converter may more quickly reach operational temperature. Additionally, if the insulated pipe is positioned where there is risk of human contact, the insulation may aid in keeping the exterior surface of the pipe cooler, thus reducing the risk of burn.
One known exhaust pipe insulator uses insulating materials, such as beads, between two layers of metallic tubes to reduce the exterior temperature of the exhaust pipe. The inner metal pipe is used to conduct heat away from its source. Another known insulator system uses a particulate based lining on the exhaust manifold to achieve some degree of thermal insulation and noise attenuation, with fiber mats filling the void spaces and providing cushioning. However, particulate-based systems are relatively non-porous, have limited less surface area, and are not very effective thermal insulators. Still another known insulation system places a particulate-based insulation liner on the exhaust manifold. Yet another known insulator system uses metal fibers in manifold-based noise abatement system for small engines. This system has higher backpressures and the metal fibers have relatively low melting point. Moreover, the metal fibers are incompatible with most catalyst materials and, since they are typically better thermal conductors, they do not provide as much thermal insulation as do the ceramic systems. Yet another insulation system incorporates a coated metallic mesh- or screen-type catalytic device; however, this device is characterized by a relatively low conversion efficiency; stacking multiple screens increases the effective conversion but likewise increases backpressure on the engine. In addition, the system offers little in the area of heat and/or noise insulation. Although these known insulated exhaust systems may offer some assistance in reducing light-off times and improving exhaust gas remediation, increasingly stringent emission standards demand further reductions in light-off time and the addition of known insulation systems alone is simply not enough to provide the requisite emissions reductions. Further, even when using these known insulators, a typical vehicle exhaust system sometimes still has to have both a pre-cat and an under-mount cat, the additions of which consume valuable space; moreover, these converters must be positioned to avoid heat hazards such as risk of burn injuries. In the case of small engines, space limitations are extremely constraining, and catalytic devices with high conversion efficiencies are much needed. Thus, there remains a need for a means of decreasing light off time, reducing noise, decreasing exhaust system surface temperature, and/or otherwise reducing pollutant emissions that does not add substantial size and weight to the exhaust system. The present invention addresses this need.